Force-sensing buttons have found recent widespread use in human interface devices such as gamepads for the entertainment consoles like the Sony PlayStation™ and Microsoft Xbox™. A conventional gamepad 100 is shown in FIG. 1. The conventional gamepad 100 comprises a housing 110, having four force-sensing triggers 120, a D-pad 130 with four force sensing buttons controlled by a left hand, four force sensing buttons 140 controlled by a right hand, and two thumbsticks 150 controlled by thumbs. The force sensing buttons comprise electronic force sensing actuators (in which the force applied to a button is sensed, rather than the binary state of a button) to provide variable force inputs to the console. In this conventional gamepad, there are twelve force sensing buttons/actuators. Typically each force sensing button/actuator output is translated in a six or eight bit value representing the force applied.
One conventional implementation for a force sensing actuator is the use of a force sensing resistor, such as those sold by Interlink Electronics (cited in information disclosure statement). However the force sensing resistor solution is too expensive for many applications where cost is an important factor. Many purchasers of gamepads and other consumer products are very price sensitive, so having a low manufacturing cost is important.
Another lower cost conventional implementation (which has been adopted by many gamepad manufacturers) is to use a resistive track printed on a printed circuit board (PCB). Printed circuit boards typically comprise a substrate, with one or more layers of copper traces on the surface or sandwiched between layers of substrate. To prevent corrosion and to prevent short circuits, the copper traces are coated with a thin film of “solder resist” except at the locations of pads or holes where components are to be soldered to the copper traces. In some cases, the copper traces may be gold plated.
In some cases, PCBs also contain resistive carbon traces printed on one or both sides of the PCB. The resistivity of such traces may vary between a few ohms/square and several kilo ohms/square. Such carbon traces may be used for a variety of purposes, including preventing corrosion of exposed copper contacts and to implement a variable resistance in combination with an external actuator or wiper.
The cost of a PCB is determined primarily by its area, the type of substrate material used, the number and size of holes in the PCB, and the number of layers of copper traces. The minimum width of the traces, and the minimum distance between traces also may significantly affect PCB cost, but the number of traces, or the percentage of the area of the PCB that is covered in copper are not significant factors affecting the cost of a PCB.
FIG. 2 shows a conventional actuator button 200 such as one used in a gamepad or other control device. The button has a carbon-impregnated domed rubber actuator, which makes contact with a resistive carbon PCB track. As the button is pressed harder, the rubber dome deforms, progressively shorting out more of the printed carbon PCB track, reducing the end-to-end resistance of the track, as shown in FIG. 2.
When the button is in the ‘rest’ position 210, it is not in contact with a carbon track 250, and resistive value of the track is shown as the resistor representation 260. When the button is gently pressed it goes to position 220, where the tip of the dome contacts the carbon track 250, and shorts across a small portion of the track 250. This is visible as the ‘shorted out’ portion of the resistor representation 265. When the button is pressed more firmly as shown in position 230, the tip of the dome deforms to become flatter and shorts out a larger portion of the track 250. This is visible as the wider ‘shorted out’ portion of the resistor representation 270. Finally, if the button is pressed hard as shown in position 240, the tip of the dome deforms to become quite flat and shorts out a much wider portion of the track 250, such that almost the entire track 250 is shorted out. This is visible as the widest ‘shorted out’ portion of the resistor representation 275.
The arrows in the drawing show the portion of the track which is not shorted out, and which is therefore resistive. The area between the arrows shows the area of the track which is shorted out. It can therefore be seen that as the rubber button is pressed harder, more of the track is shorted out, and the total resistance between the 2 ends of the track is reduced. The resistive track usually has a total resistance of a few kilo ohms, while the resistance of the conductive coating on the bottom of the rubber button is typically a few ohms at most. The resistance may be measured by placing a second resistor (for example 10K Ohms) in series with it to form a potentiometer, and measuring the output voltage from the potentiometer using an analog to digital converter (ADC).
This conventional actuator button and resistive track of FIG. 2 is somewhat less accurate than the force sensing resistor (FSR) approach, and has lower linearity. The main reason for the lower accuracy and non-linearity of the conventional actuator button and resistive track is the difficulty in printing a resistive track with a consistent resistivity along its length, and consistent resistivity from printed track to printed track. It is difficult to accurately control the thickness of the printed trace in a mass manufacturing process. However, absolute accuracy and linearity may not be important in many applications, and with calibration it is possible to give reasonably consistent results. Firmware may be used to calibrate for the non-linearity and also to calibrate for the changes in resistance as the rubber dome wears out with use. However, while the conventional actuator button and resistive track solution is less expensive than a force sensing resistor, it still costs several cents for each printed resistive trace on the PCB, and such costs can be significant in a consumer product with many force-sensing buttons (for example twelve buttons in the example of FIG. 1).
FIG. 3 shows a disassembled conventional gamepad 300, with resistive carbon traces 310, and conductive rubber dome actuators 320.
FIG. 4 shows a printed circuit board layout 400 of the conventional gamepad. The layout 400 shows resistive carbon printed traces 410, PCB traces 420, and solder resist (in this case blue, generally green in color) 430.
It would be desirable to have a less expensive force sensing button. A preferred force sensing button would be “free” (apart from the cost of the actuator itself) and provide linear sensing of force, with absolute accuracy that was consistent after calibration (low drift).